Organic Light Emitting Diode (OLED) displays were a $1.25 billion market in 2010, which is projected to grow annually at a rate of 25%. The high efficiency and high contrast ratio of OLED displays make them a suitable replacement for liquid crystal displays (LCDs) in the mobile phone display, digital camera, and global positioning system (GPS) market segments. These applications place a premium on high electrical efficiency, compact size, and robustness. This has increased the demand for active matrix OLEDs (AMOLEDs) which consume less power, have faster response times, and higher resolutions. AMOLED innovations that improve these properties will further accelerate AMOLED adoption into portable devices and expand the range of devices that use them. These performance factors are largely driven by the processing temperature of the electronics. AMOLEDs have a thin-film transistor (TFT) array structure which is deposited on the transparent substrate. Higher TFT deposition temperatures can dramatically improve the electrical efficiency of the display. Currently, glass plates are used as AMOLED substrates. They offer high processing temperatures (>500° C.) and good barrier properties, but are relatively thick, heavy, rigid, and are vulnerable to breaking, which reduces product design freedom and display robustness. Thus, there is a demand by portable device manufacturers for a lighter, thinner, and more robust replacement. Flexible substrate materials would also open new possibilities for product design, and enable lower cost roll-to-roll fabrication.
Many polymer thin films have excellent flexibility, transparency, are relatively inexpensive, and are lightweight. Polymer films are excellent candidates for substrates for flexible electronic devices, including flexible displays and flexible solar cell panels, which are currently under development. Compared to rigid substrates like glass, flexible substrates offer some potentially significant advantages in electronic devices, including:
(A) Light weight (glass substrates represent about 98% of the total weight in a thin film solar cell).
(B) Flexible (Easy to handle, low transportation costs, and/or more applications for both raw materials and products).
(C) Amenable to roll-to-roll manufacturing, which could greatly reduce the manufacturing costs.
To facilitate these inherent advantages of a polymeric substrate for the flexible display application, several issues must be addressed including:
(A) Increasing the thermal stability;
(B) Reducing the coefficient of thermal expansion (CTE);
(C) Maintaining high transparency during high temperature processing;
(D) Increasing the solvent resistance; and
(E) Increasing the oxygen and moisture barrier properties. Currently, no candidate substrate film can provide sufficient barrier properties. However, this is not a limiting factor as an additional barrier layer can be applied.
Several polymer films have been evaluated as transparent flexible substrates, including: polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), polyethersulfone (PES), cyclic olefin polymer (COP), polyarylates (PAR), polyimides (PI), and others. However, no one film can meet all the requirements. Currently, the industrial standard for this application is PEN film, which meets part of the requirements (Transmittance>80% between 400 nm and 750 nm, coefficient of thermal expansion (CTE)<20 ppm/° C.), but has a limited use temperature (<200° C.). A transparent polymer film with a higher thermal stability (glass transition temperature (Tg)>300° C.) and a lower coefficient of thermal expansion (CTE) (<20 ppm/° C.) is desirable.
Conventional aromatic polyimides are well known for their excellent thermal and mechanical properties, but their films, which must be cast from their polyamic acid precursors, are usually dark yellow to orange. Some aromatic polyimides have been prepared that can be solution cast into films that are transparent in the visible region. However, these films have significant absorbance at 400 nm. The films also do not display the required lower coefficient of thermal expansion (CTE) and are not solvent resistant (for example, F. Li. F. W. Harris, and S. Z. D. Cheng, Polymer, Vol. 37, (1996) 23, pp 5321). Polyimide films based on part or all alicyclic monomers, such as those described in Japanese patents JP 2007-063417 and JP 2007-231224, and publication by A. S. Mathews et al (A. S. Mathews et al, J. Appl. Polym. Sci. Vol. 102, (2006) pp 3316), show improved transparency. Although glass transition temperature (Tg) of these polymers can be higher than 300° C., at these temperatures the polymers do not show sufficient thermal stability.
Fiber reinforced polymer composite films, such as reported by H. Ito (H. Ito, Jap. J. Appl. Phys., Vol. 45, (2006), No. 5B, pp 4325), combine the dimensional stability of fiber glass in a polymer film, offering an alternative way to achieve a low coefficient of thermal expansion (CTE). However, in order to maintain a high transparency, the refractive indices of the matrix polymer and the fiber must be precisely matched, which greatly limits the choice of the matrix polymer. Nanoparticles have also been incorporated in polymers in an attempt to lower their coefficient of thermal expansion (CTE). However, the effect was not significant (J M Liu, et al, J. SID, Vol. 19, (2011) No. 1).
The properties of aromatic polyamides suggest they might be useful in the preparation of flexible substrates. However, the majority of polyamide are insoluble in organic solvents and, thus, cannot be solution cast into films. A few are soluble in polar aprotic solvents containing inorganic salts. Some of these have been investigated for use as flexible substrates. For example, JP 2009-79210A describes a thin film prepared from a fluorine containing aromatic polyamide that displays a very low coefficient of thermal expansion (CTE) (<10 ppm/° C.), good transparency (T %>80 between 450˜700 nm), and excellent mechanical properties. However, the presence of the inorganic salt makes film fabrication difficult. The maximum thickness of the films is also only 20 μm, because a dry-wet method must be used to remove the salt during the film preparation.